The invention relates, in general, to the electronic calibration and, more particularly, to a new and useful arrangement and technique for the electrical calibration of electrochemical sensors for determining partial pressures in physiological media.
Polarography is an electrometric method of chemical analysis that is based on the current voltage relationship at a special type of electrode. Potentiostatic and polarographic sensors operate with a constant potential. In the case of air-saturated aqueous electrolytes, the potential is preferably in the potential range 0.7 to 1.5 V relative to a reference hydrogen electrode in the same solution. In this potential range, oxidizable gases, such as CO, H.sub.2 S, NO, SO.sub.2 and the like, are oxidized without being influenced, respectively, by the currents of the oxygen reduction or oxygen evolution. The electrode current, moreover, is proportional to the concentration of the ions of the substance to be measured. The activity of the working electrode, moreover, increases with the time. The sensors must, therefore, be checked and calibrated about once a week.
If the activity of a freshly calibrated working electrode is Ao, and the activity at a measuring time t is A, then the activity ratio Ao/A yields a factor for determining the remaining output activity at a time t.
A known arrangement (see West German Pat. No. 2,431,194) for determining the partial pressure of gases which are dissolved in physiological media includes an electrode catheter having flushing channel plus flushing medium for the electrode surface. In addition, an electrical correction device is assigned to the electrode catheter. In operation, the top of the catheter is principally flushed with the flushing medium in the rhythm of a time switch or calibration, or it is surrounded by the body fluid to be measured. During the calibration phase, that is, when flushing with the flushing medium, the electrode sensitivity is determined in respect to a standard known value. During the subsequent measuring phase, the measured value is corrected by the still existing electrode sensitivity, so that a corrected partial pressure of the gas to be measured is obtained. A disadvantage of the known arrangement is that the automatic but complicated continuous correction of the measured value over the standard value must be determined anew before each measurement. For this purpose, a special flushing medium must be continuously made available.